Laser reflection/propagation through plastic

I took a cube polycarbonate sample and put a laser through it (perpendicular to a face). When light crosses both faces (front and back), some of the light will reflect while some goes through. My questions are:

1) For the light that reflects off the first face and goes straight back into the oncoming beam: does this interferance affect the output? Or what goes on when this happens? It's probably a QM question.
2) Some light that hits the 2nd face (back) will reflect back into the sample, effectively producing many many bounces inside the plastic (which are colinear). How can I predict the amount of light that comes out of the 2nd face, towards my detector?

Usually, light reflected back at a laser does not re-enter the lasing cavity due to the Brewster window at the output face. However, in those situations when light *does* re-enter the cavity, the output will become unstable.

As for #2, you can calculate the fresnel reflection coefficient, given the index of refraction of your sample and angle of incidence. Unless your sample is very inhomogeneous and/or the surface is all scratched up, the calculation should be reasonable.